Pressurized fluid tanks generally include a vent opening at the top thereof and the vent duct has an elbow connection with the top of the tank, the vent and relief valve either being interposed between the elbow and the tank or mounted in the vent duct downstream of the elbow. In either case, one known form of vent and relief valve comprises a main valve member which is pivotally mounted in the valve body and which is linked to a gas pressure actuator to swing the main valve member to open position as during filling of the tank, and to a tank pressure sensing valve which via a booster swings the main valve member to open position in the event that tank pressure builds up above a predetermined value. In a valve installation as just described a tank pressure sensing line is provided and it must have a tank pressure seal connection with the tank and with the valve body and when the valve is mounted in the vent duct there must be a tank pressure seal between the elbow and valve body in addition to the tank pressure seal connection of the elbow to the top of the tank.
Such known vent and relief valve construction is quite heavy, complex and expensive and, in the case of a valve mounted between the tank and the elbow, additional vertical clearance space is required above the tank.